Tokyo Electric Power Co. said one of the reactor cores at its stricken Fukushima Dai-Ichi nuclear plant is more seriously damaged than previously thought, setting back the utility’s plan to resolve the crisis.

The U.S. Nuclear Regulatory Commission released transcripts that officials said supported Chairman Gregory Jaczko’s recommendation last year to evacuate Americans from within 50 miles of Japan’s crippled reactor.

The U.S. Senate confirmed Allison Macfarlane to lead the Nuclear Regulatory Commission as it grapples with aging reactors, long-term atomic waste disposal and a response to last year’s disaster in Japan.